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Bald Eagle
Haliaeetus leucocephalus
Bald Eagle
Behaviour
Bald Eagle
Breeding Behaviour
Breeding Biology
Bald Eagles
mate monogamously probably with the same mate each year if
successful (Buehler 2000). On the Pacific Coast, pair bonds
likely remain intact because eagles stay on the territory for
most of the year. It is unclear where pairs on the Pacific
Coast select mates but it is likely on the nesting territory.
Adults defend against other eagles in the vicinity of the
nest. It is uncertain what they are defending; the nest, a
fertile mate, the nestlings or the food supply. The defense
includes territorial warning calls and pursuits.
Egg laying
occurs in March in southern British Columbia and April in
southeast Alaska. A nest can be built in a few weeks but most
are assembled over two or three months. Both adults gather
nest sticks and the female places them in position. Eagle
nests range from about a one meter deep pile of branches to
massive structures several meters deep. Branches provide shape
and twigs, moss or grass line the cup to hold the eggs. Two
eggs but sometimes 1 or 3, are laid in two or three days
intervals and incubated for 35-40 days. Eggs are ivory white
in colour with no markings and they measure about 7.3 cm long
and 5.5 cm wide (Buehler 2000). Females do most of the
incubation. Eaglets are covered with a silvery gray down upon
hatching. Not much is known about growth of eagles especially
on the Pacific Coast. Elsewhere they achieve maximum growth
when they are 3 to 4 weeks old. Males attain 102g/d and
females 130g/d (Buehler 2000). The eaglets remain in the nest
for about 2 months with males growing their feathers and
learning to fly more quickly than females. Sibling mortality
is most frequent in the first few weeks after hatching (Bortolotti
1986). In Saskatchewan where eagles have been studied closely,
the third chicks were rarely fed and often starved (Gerrard
and Bortolotti 1988). Mothers brood nearly continuously until
the chicks are 4 weeks old (Buehler 2000). By 5 weeks, the
parents roost in trees nearby. Third eggs are thought to be
insurance eggs in the oft chance that one of the other eggs is
lost; if no eggs are lost, the hatched young is discarded so
that the food can be directed to the first two young with the
greatest survival chances. Both parents feed the chicks that
they transport to the nest. Bits of food are torn into small
pieces and placed in the mouths of the young. Fathers do most
of the feeding of chicks in the first two weeks after
hatching. From 3 to 4 weeks, both parents equally provision
the young. At 6 weeks the young can feed themselves from food
dropped in the nest (Buehler 2000). Nestlings take to the air
as early as 8 weeks of age but some require as much as 14
weeks. Immature eagles remain with their parents for several
weeks after leaving the nest and likely follow them to
spawning streams. Young eagles are not efficient hunters when
they are first beginning to feed themselves and they will
often scavenge dead prey. Spawning salmon likely provide the
food source they require and explain why eagle populations
have remained high along the Pacific Coast.
Bald Eagle
Feeding Behaviour
Bald eagles eat by killing victims, scavenging
carcasses and stealing food obtained by other eagles and
birds. On the Pacific Coast, some eagles have specialized on
hunting ducks or seabirds. They repeatedly stoop sometimes as
pairs on diving ducks which succumb from exhaustion, and on
adult seabirds at sea or their young at nesting colonies. They
also kill adult herons on nests, young herons recently out of
the nest, heron chicks and eggs in nests (Butler 1997). Eagles
catch fish by snatching them from the water occasionally
becoming waterborne themselves. Eagles are strong but awkward
swimmers. Much of the hunting begins with the eagle soaring
overhead or from a perch. Once a victim is seen, the eagle
will launch a direct rapid flight. Other birds recognize the
intention of these flights and quickly avoid the eagle.
Victims are caught with the talons and carried to a nearby
post to be dispatched with the sharp hooked beak.
Bald eagles in captivity required at least
425.5 kJ of food energy each day when the air temperature was
5oC (Stalmaster and Gessaman 1982). About 75% of
the food was assimilated. These authors estimated that a 4.5
kg captive eagle required 13 salmon to survive for 90 days in
winter. A free-living eagle in Washington State was estimated
to need 490 g of salmon each day to maintain its weight.
Adults needed 522g/day, sub adults (2-5 yrs old) needed 410
g/d and immatures (<1yr) required 459g/d (Stalmaster and
Gessaman 1984).
Bald Eagle
Vocalizations
The calls of adult and immature bald eagles are
weak and scratchy sounding. Nestling utter weak peeping noises
and by one month begin to sound like those of the adult.
Immatures give a cheeping noise when they beg to parents for
food.
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